1. Field of the Invention
The present invention relates to recording elements and, more particularly, to a recording element having excellent coloring speed.
2. Brief Description of the Invention
Recording elements relying on a substantially colorless color former which is brought into contact with an acidic substance (the color developer) to produce color are already well known. Representative examples are pressure-sensitive copying paper, heat-sensitive recording sheets, electric current-applying recording sheets and the like, which are described in detail in U.S. Pat. Nos. 2,712,507, 2,730,456, 2,730,457, 3,418,250, 3,432,327, 3,981,821, 3,993,831, 3,996,156, 3,996,405, 4,000,087, etc.
The color developers used in these recording elements are clay minerals such as acidic clay, bentonite, kaolin, etc., organic acids or salts thereof such as isopropenylphenol dimer, novolak, metal-treated novolak, di-tertbutylsalicylic acid, di-.alpha.-methylbenzylsalicylic acid zinc salt, etc.
The color formers in these elements are compounds that are usually substantially colorless and possess a partial structure such as a lactone, lactam, sultone, spiropyran, esters, amides, etc., and are brought into contact with color developers, whereby these partial structures are opened or cleaved. Specifically, these compounds are exemplified by crystal violet lactone, benzoyl leucomethylene blue, marachite green lactone, rhodamine B lactam, 1,3,3-trimethyl-6'-ethyl-8'-butoxyindolinobenzospiropyran, etc.
These color developers and color formers are occasionally employed in combinations of two or more thereof depending upon utility. The color formers are dispersed or dissolved in a solvent and microencapsulated in order to be stably stored until they are employed in recording elements.
In general, aromatic compounds such as hydrogenated terphenyl, alkylated diphenyl alkanes, alkylated triphenyl dimethane, etc., aliphatic compounds such as kerosine, chlorinated paraffin, cotton seed oil, etc., are employed as solvents. For these solvents, the properties have been hitherto required such that (1) the solubility of color formers therein is excellent and (2) the coloring density is high, etc.
Further, organic acids or salts thereof have been investigated as color developers since the stability of colored images with the passage of time, humidity resistance and so on is superior. In the case of using organic acids or metal compounds thereof, however, the image density immediately after color formation is poor. The present invention is directed to overcoming this disadvantage.